The present invention relates to a radiation detecting device and, more particularly, to a solid state radiation detector for detecting radiation rays by using interaction occurring when radiation rays entering the detector pass through the detector.
A "radiation detector" is a general name of devices for detecting radiation rays by utilizing a phenomenon occurring when radiation rays, such as .alpha. rays, .beta. rays, and .gamma. rays, and elementary particles, atomic nuclei, protons, and the like, which move with energy equivalent to or higher than these rays, pass through a specific material. Various radiation detectors are proposed, and each detector can be used for various purposes by changing its performance or shape. The radiation detectors may be classified into a gas detector, a liquid detector, and a solid state detector in accordance with materials for generating electrical signals upon detection of radiation rays.
In recent radiation detectors which are incorporated in various industrial products such as a radiation CT scanner (computed tomography scanner), rolled plate thickness measurement machine, and the like, a solid state radiation detector is primarily used. This is because, the solid state radiation detector is compact in size, has excellent reliability, and can be manufactured with relatively low cost. However, the conventional solid state radiation detector has poor conversion efficiency of radiation rays into signal charge carriers. As a result, it is difficult to improve a detection sensitivity to a required level.
For example, according to a conventional multichannel radiation detector suited for the X-ray CT scanner, the radiation detector has a scintillator and a semiconductor radiation detecting element in a collimator for defining each channel. The semiconductor radiation detecting element comprises a silicon substrate which is sandwiched between a Schottky junction metal layer for forming a depletion layer as a sensitive section at an interface between itself and the substrate, and an ohmic contact metal layer for outputting an electrical signal. Incident radiation rays are directly detected by the semiconductor radiation detecting element. The incident radiation rays passing through the detecting element are then guided to the scintillator, and the scintillator emits light. The scintillation light is detected by the semiconductor radiation detecting element.
With this prior art, the incident radiation rays are subjected to two-step detection, i.e., direct detection by means of the semiconductor radiation detecting element, and indirect detection using the scintillation light. Thus, it may be considered that the conversion efficiency of the radiation rays is relatively good. However, since the depletion layer of the semiconductor detecting element as the sensitive section is formed only on one surface of the substrate, the conversion efficiency of the radiation rays still remains low. Therefore, its detection sensitivity is low. If the size of the semiconductor detecting element is increased to several centimeters, the detection sensitivity may be improved to some extent. However, this renders the detector undesirably bulky.
As another prior art device, a semiconductor radiation detector having a pin junction type amorphous silicon layer is known. More specifically, an n-type amorphous silicon layer, an intrinsic amorphous silicon layer, a p-type amorphous silicon layer, and a transparent conductive electrode layer are sequentially stacked on a substrate. By this detector, when incident radiation rays pass through the pin junction type amorphous silicon layer, electron-hole pairs are produced in the silicon layer by a photovoltaic interaction. Since the electron-hole pairs can move about in the silicon layer, if an appropriate electric field is applied to the silicon layer, the electron-hole pairs can be read out. Therefore, electrical pulses of a detection signal can be obtained.
With the detector of this type, however, the conversion efficiency of the radiation rays at the pin junction type amorphous silicon layer is not so high, and the radiation ray detection sensitivity is low. In order to improve the detection sensitivity, the amorphous silicon layer portion must have an extremely large thickness (e.g., the thickness of the intrinsic amorphous silicon layer must be increased to several centimeters). This is because if the thickness of the amorphous silicon layer portion is commonly set to be several micrometers, radiation rays cannot be absorbed by the amorphous silicon layer portion, and almost all the rays pass through the silicon layer portion. In this manner, it is difficult even for latest film formation technology to allow the manufacture of a uniform amorphous silicon layer having an extremely large thickness. If it can be manufactured, carriers produced in the amorphous silicon layer cannot be derived as electrical detection signals since carrier lifetime and carrier mobility of the carriers inside the silicon layer are low.